The present invention relates to a semiconductor memory device and a method of operating the same; and particularly relates to a semiconductor memory device capable of improving characteristics of threshold voltage distribution of memory cells and a method of operating the same.
Recently, the number of memory cells connected to one word line has increased according to enhanced reliability of a semiconductor memory device such as flash memory devices. Length and load of the word line increase as the number of the memory cells increases.
A row decoder may supply a voltage to the word line during a program operation. However, a program voltage level applied to a control gate of a memory cell, which is close to the row decoder, is different from that applied to another control gate of a memory cell, which is far from the row decoder, as the load of the word line increases.
Furthermore, because of different program voltage levels, increased level of a threshold voltage in the memory cell which is near the row decoder is different from that of a threshold voltage of the memory cell which is far from the row decoder. Accordingly, threshold voltage distribution of the memory cells becomes wide.
The wide distribution of threshold voltage may deteriorate performance of program, because the time required for a program operation becomes long when an additional program pulse is applied to the word line so as to increase the threshold voltage of the memory cell which is far from the row decoder. A method of improving the threshold voltage distribution of the memory cells without deteriorating performance of the program operation has been required.